With the recent progress in digital technology, it has become customary that analog signals such as audio or video signals are sampled at certain intervals and each sampled value is quantized and coded for providing pulse code modulated signals (so-called PCM signals) for transmission (or occasionally recording or reproduction).
It is known in general that, converting the analog signals into PCM digital signals for transmission, the higher the sampling frequency, the wider the bandwidth of the analog signal that can be transmitted and, the larger the number of bits for quantization, the larger the dynamic range of the output signal. Therefore, when it is desired to effect a digital transmission of the original analog signal with a high fidelity, viz. with a broad frequency range and a large dynamic range, it is necessary to use a high sampling frequency and a large number of bits for quantization, with the result that the number of bits transmitted in a unit time or the bit rate is necessarily increased.
However, the bit rate is limited not only by the properties of the transmission medium or occasionally the recording medium, but also by the digital signal processing speed available at the transmitting and receiving sides or occasionally at the recording and reproducing sides. Also in consideration of the cost performance and economy in supplying PCM signal recording and reproducing devices, it is critical to effect a high-quality signal transmission and occasionally recording or reproduction at as low a bit rate as possible.
For signal transmission at a relatively low bit rate and a relatively large dynamic range, it is known to use a differential PCM system or an adaptive differential PCM system. These systems, however, are susceptible to error propagation and, if it is desired to insure an acceptable error corrective ability, the effect of bit rate reduction may not be achieved because of increased redundancy. In the differential PCM system, the dynamic range larger than that possible with the conventional or straight PCM system can be obtained at a smaller number of bits for quantization in the low to medium frequency range of the input signal up to one-sixth of the sampling frequency. However, with the high frequency range of the input signal, the dynamic range of the differential PCM system may become lower than that of the straight PCM system. In addition, in the differential PCM system, the effect of transmission error tends to be demonstrated markedly while the sound quality may be degraded due to increased distortion factor.
In introduction of the adaptive PCM system, in which the bit length per word is reduced by having the quantizing step width changed in accordance with the input signal for reducing the bit rate without affecting the dynamic range, since the input digital signals with a longer bit length per word are adaptively processed for conversion into words of a shorter bit length or re-quantizated, there is such a defect as the increased quantization noise.
In view of such aspect as above described, it is therefore an object of the present invention to provide a method and an apparatus for transmission of digital signals, according to which the error propagation can be terminated in a shorter time, the effect of the dynamic range expansion is drastically improved by this means a coefficient for suppressing the error propagation is increased and the adaptive processing is intensified, and a high error corrective ability is achieved as the redundancy is kept low for the sake of a simple code construction.
It is another object of the present invention to provide a method and an apparatus for transmission of digital signals according to which a transmission mode best suited for transmission of input signals, for instance, one of the straight PCM, differential PCM and the additive PCM modes is selected for each of the temporally divided data blocks so as to provide the maximum transmission efficiency.
It is a further object of the present invention to provide a method and an apparatus for transmission of digital signals whereby a signal waveform exhibiting rapid changes in amplitude with time, may be transmitted, or recorded and/or reproduced, efficiently with a low bit rate.
It is another object of the present invention to provide a method and an apparatus for transmission of digital signal according to which, in view of the masking effect of hearing sense of the human, the quantization noise spectrum is changed as a function of the frequency spectrum of the input signals for providing the maximum masking effect so as to be able to reduce the apparent or auditory noise.